1. Field of the Invention
The present invention relates to a technology for adjustment of a zero point in a level of an image signal output from an imaging device.
2. Description of the Related Art
Imaging devices that include charge-coupled devices (CCD) to take digital images are known in the art. With such charge-coupled device (CCD) imaging devices, when an image of a target object is taken in a dark environment, for example, in the presence of backlight, the brightness of the obtained image is not adequate. One approach to create an image with adequate brightness, as disclosed in Japanese Patent No. 2807928, is to increase the gain of a gain control amplifier (GCA) circuit using an automatic gain control (AGC) circuit and amplify the image signal output from the CCD imaging device using the GCA circuit that has the increased gain.
Output of an imaging sensor, such as a CCD imaging device, contains a certain amount of noise; therefore, an image that is created based on the output also contains noise. To reduce noise, if the output level of the imaging sensor is lower than a threshold, the zero-point level of an analog-to-digital converter (ADC) circuit is adjusted by performing a black-level control (BLC) so that a signal having a level equal to or lower than the threshold cannot be output from the imaging sensor.
More precisely, if the output level of the imaging sensor is equal to or lower than the threshold, the output from the imaging sensor is cut off. Therefore, when an image is taken in a dark environment where illuminance is equal to or lower than the threshold (hereinafter, “sensitivity limit”), the obtained image is too dark to be seen by the naked eye.